1. Field of the Invention
This invention generally relates to playback apparatus for deriving signals from an optical video disk. More particularly, the invention is directed to a supporting system used in such playback apparatus for controlling a mirror for tracking and time base error correction.
2. Description of Prior Art
In playback apparatus for optically reproducing information recorded in the form of minute pits spaced apart on a spiral track on a flat disk, a beam of light from a source is directed along an optical path and is focused on the disk. The light beam is either reflected by or transmitted through the disk. In either case, the beam is modulated by the recorded pits, and the modulated beam is detected by a photodetector, and the output of the photo-detector is an electrical signal that is a reproduction of the information signal recorded in the minute pits.
This type of playback apparatus utilizes no mechanical track-following means, and so the recorded information can be reproduced without using a contact mechanism to follow the track. Because of this characteristic, various kinds of servo system, for example, the focus control servo, the tracking control servo, and the time base error control servo have been developed to keep the light beam directed at the track and even at the proper point along the track at each instant.
As described by Robert Adler in "An Optical Video Disc Player for NTSC Receivers", Transactions BTR, Aug. 1974, the beam can be directed precisely at the right point along the track instant by instant by a servo mirror system incorporating a two-axis mirror inclined at an angle to the optical path and thereby defining an intersection between two parts of the path. The mirror is affixed to a magnetic member and is pivotally supported by a pointed supporting member. Two pairs of driving coils are placed around this mirror with the axis of each pair perpendicular to the axis of the other pair. These coils can be energized by currents to create magnetic fields to interact with the magnetic member affixed to the mirror to pivot the mirror about its two axes and thereby direct the reflected light exactly to the right point along the track.
The first pair of coils controls the angle of inclination of the mirror in one direction so that the light beam can be moved in the radial direction of the disk to correct the tracking error. The second pair of driving coils controls the angle of the mirror in the perpendicular direction so that the light beam can be moved in the tangential direction of the disk, that is, longitudinally along the track to correct the time base error. By using this two-axis mirror, only one servo mirror need be used; one of two servo mirrors for tracking error correction and time base error correction can be omitted.
As is well known, the inclined angle of the mirror must be precisely controlled and careful, minute adjustments are necessary, particularly when the disk rotates at a high speed. If the inclined angle of the mirror is affected by other causes than by the drive coils, stable servo operation cannot be attained and correction of either the tracking error or the time base error is difficult to attain.
In this two-axis servo mirror system, the mirror is pivotally supported on the pointed tip of the supporting member and is likely to rotate about an axis through this point because the friction between the mirror and the supporting member is very small. But if the mirror can be supported so that its surface is perpendicular to the axis of the supporting member in the neutral state, even if the mirror is rotated on the axis, the deviation of the landing spot of the light on the disk in both the radial and tangential directions can be negligible. It is very difficult to maintain the reflective surface of the mirror precisely perpendicular to the axis of the supporting member, due primarily to the difficulty of controlling the mechanical accuracy of the position or the shape of the mirror and the supporting member. Therefore the mirror is likely to be supported in an inclined condition, even in the neutral state. In this condition if the mirror is rotatable on the axis of the supporting member, there are infinitely many stable surfaces in the mirror, so that the light is likely to deviate from the proper landing spot on the disk. Because of this, rotation of the mirror on the supporting member causes the tracking control and the time base control to be very unstable.